Interconnect substrates for portable and wireless equipment need to be thin and light. Because this type of equipment combines digital and RF circuitry, the interconnect medium needs to have a large interconnect capability (cm/cm2of wire) for the digital circuitry, and also have the capability to provide controlled impedance, low loss transmission lines for critical RF circuitry.
Traditionally, these needs have been met by providing buried stripline structures, in which X and Y signal layers are sandwiched between power and ground planes, which provide the AC ground reference for the transmission lines, as well as low impedance power distribution for all circuitry. A circuit board may have 4 or 6 layers of metal, with outermost pad layers for attaching components, which may also carry sections of power distribution planes or signal lines. These pad layers may form the reference planes, or the planes may be carried on another inboard set of metal layers.
A problem with using any form of buried stripline transmission line is that, once the manufacturing process has determined a reasonable thickness for the layers of dielectric insulator, the impedance of the line and the loss of the line are no longer independent. The impedance is determined by the width and height of the metal line, the distance between the line and the power and ground planes, and the dielectric constant of the insulating layers. The manufacturing process usually determines the thickness of conducting and insulating layers and the dielectric constant of the insulating material. For portable equipment, thin dielectric layers are desirable.
Thus, to achieve a particular characteristic impedance, of the line, which typically might be 50 ohms, the only variable left to the designer is the width of the line. However, setting the width of the line also establishes the cross-section and thus the resistance of the line, and therefore the loss. Circuit designers have no choice but to live with the resulting loss, which, with thin dielectrics and narrow lines, can be considerable.
U.S. Pat. No. 5,410,107 issued Apr. 25, 1995 and U.S. Pat. No. 6,255,600 issued Aug. 3, 2001, to this inventor (both incorporated herein by reference), describe examples of inter-connected mesh plane system (IMPS) power distribution topologies, in which X and Y metal line segments, on different physical layers, are connected by vias to form electrical power distribution planes. Signal lines, also consisting of X and Y segments, are included between power and ground segments. The complete structure, which may include any number of physical X and Y layer pairs, can provide low impedance planar power distribution for multiple voltages, as well as dense wiring capability. This capability has been demonstrated in as few as 2 metal layers.
It has also been demonstrated that the signal lines are controlled impedance transmission lines, which function well into the multi-GHz frequency range. Return currents, rather than being carried by solid planes which underlie the signal paths, as in the traditional microstrip or stripline configurations, instead are carried on the parallel, coplanar power and ground segments adjacent to the signal line segments. When an X signal segment connects to a Y signal segment on a different metal layer, the return current follows the signal current path through the vias which connect the power and ground segments.
In a mesh plane configuration, the impedance of a transmission line is mainly determined by the width of a signal conductor segment, and the distance between this conductor and the adjacent ground return path conductors (which may be power and/or ground conductors, which are at AC ground potential). Because the orthogonal conducting segments on adjacent metal layers cannot carry current in the direction of signal propagation, the dielectric layer thickness has much less effect on the characteristic impedance of the transmission line. The present invention takes advantage of the discovery that the mesh plane configuration provides an additional degree of freedom in design, which can be used to independently determine the impedance and loss of the line.
More particularly, the width of a signal line intended to carry RF current, for example, and the width of adjacent ground and/or power lines can be increased compared to the width in a basic configuration, to reduce current losses. In addition, the space between the signal line and the adjacent lines is increased compared to a basic configuration, to obtain or maintain a desired transmission line characteristic impedance.